In order to meet exhaust gas emission standards, the exhaust emitted from internal combustion engines is treated prior to release into the atmosphere. Exhaust is passed through a catalytic element to remove undesirable gaseous emission components such as unburned hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). In addition to the gaseous components, exhaust gases, especially diesel engine exhaust, also contain solid particulate matter. The main component of solid particulate matter (PM) is carbon soot. Other PM includes soluble organic fractions (SOF) and sulfate (H2SO4) that are usually absorbed on carbon particles and contribute to the total mass of the PM. A particulate filter, which has the capability to trap or adsorb PM, can be used with compression ignition engines to prevent PM from exiting the tailpipe. Carbon particles and SOF are stored in the filter and then burned so that the filter is regenerated and able to again store more PM. In an uncatalyzed diesel particulate filter, the filter gradually fills with particulate matter. Two things may happen when the filter is loaded with a significant amount of particulate matter. First, the backpressure of the exhaust line may become too high and cause the engine to stall. Second, the engine load may increase significantly and generate enough heat so that the exhaust temperature exceeds the ignition temperature of carbon. The trapped carbon will then automatically combust and the filter will self-clean. In the later case, however, although the engine will still run, it may cause a significant loss of power, and a high fuel economy penalty due to that high backpressure generated from exhaust line. It is thus desirable to reduce the ignition temperature (also called “lightoff” temperature in engine exhaust after-treatment industry) to a lower level so that the soot can continuously or periodically burn off at the normal engine operating conditions.
To reduce the ignition temperature of diesel soot and to lower the temperature for diesel particulate filter regeneration, several catalyst systems have been developed for use in catalyzed diesel particulate filters. Eutectic salts such as KVO3 (potassium vanadate) and CsVO3 (cesium vanadate) are efficient diesel soot oxidation catalysts that reduce soot ignition temperatures and increase soot combustion rates. Other prior approaches include the use of catalysts containing noble metals and various other components such as alkaline earth metal oxides, cerium components, and copper and iron compounds, for example. While several combinations of catalyst materials have been disclosed, there has been little or no emphasis on methods of making catalyzed diesel particulate filters with improved catalytic performance and stability.
There thus remains a need for catalytic device for catalytic elements, for catalyzed diesel particulate filters, and for methods of making catalyzed diesel particulate filters that have good catalytic properties and stability.